If you're in London, now! I'm in London and was told NCT books up well before 20 weeks - I booked early (with the option to cancel if things don't go to plan). Have a look around asap - courses usually start when you're around 30+ weeks.

My nhs ones were booked up when I rang at 24 weeks managed to get a couple on cancellation. We booked our Nct classes at 12 weeks but I know that some people on our course had booked late. Id do it this week if I were you x